Lubricants in commercial use today are prepared from a variety of natural and synthetic base stocks admixed with various additive packages depending upon their intended application. The base stocks typically include mineral oils, polyalphaolefins (PAO), gas-to-liquid base oils (GTL), silicone oils, phosphate esters, diesters, polyol esters, and the like.
A major trend for passenger car engine oils (PCEOs) is an overall improvement in quality as higher quality base stocks become more readily available. Typically the highest quality PCEO products are formulated with base stocks such as PAOs or GTL stocks.
Lubricants are composed of a base stock and additives. Additives are added to the base stock either to enhance an already-existing property, such as viscosity, of base oil or impart a new property, such as detergency, lacking in the base oil. The lubricants are designed to perform a number of functions, including lubrication, cooling, protection against corrosion, and keeping equipment components clean by suspending originally insoluble contaminants in the bulk lubricant. While for automotive applications, all functions are important, suspending the insoluble contaminants and keeping the surface clean are the most critical. This is mainly achieved by the combined actions of detergents and dispersants.
Dispersants are metal-free and hence they do not form ash. The goal of the dispersant is to keep insoluble particles suspended in the bulk lubricant. The dispersants suspend deposit precursors in oil in a variety of ways. These comprise including the undesirable polar species into micelles; associating with colloidal particles, thereby preventing them from agglomerating and falling out of solution; suspending aggregates in the bulk lubricant, if they form; modifying soot particles so as to prevent their aggregation, as the aggregation will lead to oil thickening, a typical problem in heavy-duty diesel engine oils; and lowering the surface/interface energy of the polar species in order to prevent their adherence to metal surfaces.
Conventional dispersants used in PCEOs are prepared via functionalization of polyisobutylene (PIB) of different molecular weights with maleic anhydride or phenol, followed by reaction with polyamines. See Lubricant Additives, Chemistry and Applications, edited by L. R. Rudnick, 2009.
A dispersant molecule consists of three distinct structural features: a hydrocarbon group, a polar group, and a connecting group or a link. The hydrocarbon group is polymeric in nature and typically ranges from molecular weight 600 to 7000. While various polymers such as PIB or polyalphaolefins are used to make dispersants, PIB is most common. The polar group is usually an amine and is basic in character. The class of amines most commonly used to synthesize dispersants are polyalkylenepolyamines, such as diethylenetriamine, triethylenetetramine, and tetraethylenepentamine. The polar group is attached to the polymer via a linking group such as maleic anhydride.
Since it is not easy to attach the polar group directly to the hydrocarbon group, generally a polar group is attached to the hydrocarbon group via a linking group. Alkenylsuccinic anhydride is synthesized by reacting an olefin, such as PIB, with maleic anhydride. Succinimide group results when a cyclic anhydride is reacted with a primary amine. Alkenyl succinic anhydride is the precursor for introducing the succinimide connecting group in dispersants. The polyamine is then reacted with the anhydride to obtain succinimide.
The conventional dispersants prepared via functionalization of PIB of different molecular weights with maleic anhydride or phenol, followed by reaction with polyamines, work well for traditional lubricant formulations. In many automotive engine lubricant formulations, 3 to 15 wt. % of dispersant is used, the highest amount of all additives used in the formulation.
Newer lubricants are formulated to meet higher fuel economy standards, longer oil drain intervals, and more operating severity. This trend calls for the use of even higher concentration of dispersants and lower finished lubricant viscosity. Using a higher amount of PIB-based dispersants increase the finished lubricant viscosity, making the formulation difficult to stay within lower viscosity grades, such as 0W20 or 0W30, for the fuel economy.
Alternatively, formulators are pressed to use even lower viscosity base oil to achieve these fuel-efficient viscosity grades, thus risking other undesirable results, such as higher volatility, reduced lubricant oil film and reduced wear protection, and the like. Thus, there is a need to mitigate the viscosity increasing effect by PIB-based dispersants.
Additional references of interest include: EP 490454; WO 8701722; U.S. Pat. No. 5,616,153; U.S. Application Publication No, 2003/0171225; DE 19508656; WO 9402572; U.S. Pat. No. 5,319,030; and U.S. Application Publication No. 2003/013620.
There is also a need to develop chemical modification routes, especially non-maleic anhydride based, and where the vinyl double bond is more reactive than the traditional vinylidene terminus available in PIB macromers. The present disclosure provides many advantages in meeting these needs, which shall become apparent as described below.